Basic knowledge of Corrosion.
Corrosion—the deterioration of a metal or its properties—attacks every component at every stage in the life of every oil and gas field.
INDEX:-
- Corrosion
- Why do metals corrode?
- Corrosion Consequence:
- Types of Corrosion and It's how to Control?
- What causes corrosion?
- Is Corrosion and rusting same?
- What is corrosion give three examples?
- What is the most common cause of Corrosion?
Corrosion
Corrosion a chemical or electrochemical reaction between a material
and its environment that produces a deterioration (change) of the
material and its properties.
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Why do metals corrode?
Most metals are found in nature as ores. The manufacturing process of converting these ores into metals involves the input of energy.
During the corrosion reaction the energy added in manufacturing is released, and the metal is returned to its oxide state.
Metal Ore Reduction (add Electron) Metal Oxidation (strip electron) Corrosion Products.
Corrosion Consequence:
Types of Corrosion and It's how to Control?
- Uniform Corrosion
- Galvanic Corrosion
- Intergranular corrosion
- Crevice Corrosion
- Pitting Corrosion
- Selective Leaching
- Erosion-corrosion
- Stress Corrosion cracking
- Uniform Corrosion
• Selection of a more corrosion resistant alloy (i.e. higher alloy content or more inert alloy)
• Utilize coatings to act as a barrier between metal and environment.
• Modify the environment or add chemical inhibitors to reduce corrosion rate.
• Apply cathodic protection.
• Replace with corrosion resistant non-metallic material,
2. Galvanic Corrosion
- Galvanic Corrosion is an electrochemical action of two dissimilar metals in the presence of an electrolyte and an electron conductive path.
- It occurs when dissimilar metals are in contact.
Control
• Use of galvanically compatible
materials
• Avoid unfavorable area effects of a small anode and large cathode
• Use of electrical insulation between dissimilar materials
3. Intergranular corrosion
Intergranular corrosion is an attack on or adjacent to the grain boundaries of a metal or alloy.
A highly magnified cross section
of most commercial alloys will show its granular structure.
This structure consists of quantities of individual grains, and each of these tiny grains has a clearly defined boundary that chemically differs from the metal within the grain center.
Control
• Heat treatment of alloy to remove phases from grain boundary regions which reduce corrosion resistance (i.e. solution annealing).
• Use modified alloys which have eliminated such grain boundary phases through stabilizing elements or reduced levels of impurities.
4. Crevice Corrosion
Crevice Corrosion is an intense localized corrosion frequently occurs
within crevices and other shielded areas on metal surfaces exposed to
corrosives.
This type of attack is usually associated with small volumes of
stagnant solution caused by holes, gasket surfaces, lap joints, surface
deposits, and crevices under bolt and rivet heads.
Control
• Redesign of equipment to eliminate crevices.
• Close crevices with non-absorbent materials or incorporate a barrier
to prevent of moisture penetration into crevice.
• Prevent or remove builds-up of scale or solids on surface.
• Use of one-piece or welded construction versus bolting or riveting.
• Select more corrosion resistant or inert alloy.
5.Pitting Corrosion
Pitting is a form of extremely localized attack that results in holes in the metal.
These holes may be small or large in diameter, but in most cases they are relatively small. Pits are sometimes isolated or so close together that they look like a rough surface.
Control
• Choose the material most appropriate for the service conditions.
• Avoid stagnant zones and deposits
• Reduce the aggressivity of the medium (using inhibitors)
• Maintain the protective film of the material
• Use cathodic protection.
6.Selective Leaching
Selective Leaching is the removal of one element from a solid alloy by corrosion
processes. The most common example is the selective removal of zinc in brass
alloys (dezincification). Similar processes occur in other alloy systems in which
aluminum; iron, cobalt, chromium, and other elements are removed
Control
• Select “inhibited” versions of copper alloys.
• Use alternative materials that are not susceptible to dealloying in the
environment(s)
• Reduce severity of environment through environmental control or addition of
effective chemical inhibitors
• Cathodic protection
• Use of coating to act as a barrier between the environment and the alloy
7.Erosion-corrosion
Erosion-corrosion is a description for the damage that occurs when particle erosion and/or high flow velocity contributes to corrosion by removing protective films or scales or otherwise accelerating the corrosion rate.
Control
• Changes in shape, geometry, and materials can help mitigate erosion and erosion-corrosion. Examples include increasing the pipe diameter to reduce velocity.
• Improved resistance to mechanical erosion is usually achieved by increasing component hardness.
• Heat exchangers utilize impingement plates and occasionally tube ferrules.
• Ensure proper operation to avoid water droplets in the steam system.
• Use abrasion resistance coating.
8. Stress Corrosion cracking
SCC is Cracking caused by the simultaneous presence of tensile stress and a specific corrosive medium. Usually lead to unexpected sudden failure.
Examples: (Chloride SCC, Carbonate SCC, Caustic SCC, Ethanol SCC, HF SCC
and Polythionic acid SCC).
• Use resistant material
• Properly apply coating if applicable
• Residual stress release application when applicable
• Design to avoid stagnant conditions of species causing SCC
• Proper application of NDE and inspection techniques for early detection of cracks.
What causes corrosion?
The main cause of corrosion on most of the metals is its exposure to weather conditions. Therefore if you keep your metal objects outdoors in the open, they are going to corrode. Since they are exposed to environmental elements such as water, wind, and moisture, they will be oxidized quickly.
Is Corrosion and rusting same?
Corrosion is the process of deterioration of materials as a result of chemical, electrochemical or other reactions. Rusting is a part of corrosion and is a chemical process which results in the formation of red or orange coating on the surface of metals.
What is corrosion give three examples?
The formation of rust on iron, tarnish on silver, and the blue-green patina that develops on copper are all examples of corrosion. Corrosion is defined as the degradation of metals due to an electrochemical process.
What is the most common cause of corrosion?
The most common kinds of corrosion result from electrochemical reactions. General corrosion occurs when most or all of the atoms on the same metal surface are oxidized, damaging the entire surface. Most metals are easily oxidized: they tend to lose electrons to oxygen (and other substances) in the air or in water.
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